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Abstract:

A lithium-metal-oxide positive electrode having a layered or spinel
structure for a non-aqueous lithium electrochemical cell and battery is
disclosed comprising electrode particles that are protected at the
surface from undesirable effects, such as electrolyte oxidation, oxygen
loss or dissolution by one or more lithium-metal-polyanionic compounds,
such as a lithium-metal-phosphate or a lithium-metal-silicate material
that can act as a solid electrolyte at or above the operating potential
of the lithium-metal-oxide electrode. The surface protection
significantly enhances the surface stability, rate capability and cycling
stability of the lithium-metal-oxide electrodes, particularly when
charged to high potentials.

Claims:

1. A positive electrode for a non-aqueous lithium cell comprising
lithium-metal-oxide particles, the surface of the particles comprising a
lithium-metal-polyanionic material, the general formula of which
comprises at least one lithium cation, at least one other metal cation,
and at least one polyanion.

2. The electrode of claim 1, wherein the lithium-metal-oxide particles
comprise a spinel-type structure, a layered-type structure, or a
combination thereof.

3. The electrode of claim 2, in which the layered-type structure comprises
one or more compounds represented by the general formula,
xLi2M'O.sub.3.(1-x)LiMO2, in which M' comprises one or more
metal ions with an average tetravalent oxidation state, and M comprises
one or more metal ions with an average trivalent oxidation state.

4. The electrode of claim 3, in which M' comprises Mn, and M comprises one
or more of Mn, Ni and Co.

6. The electrode of claim 1, wherein the lithium-metal-polyanionic
material is a lithium-ion conductor that operates at or above 4 V versus
metallic lithium.

7. The electrode of claim 6, wherein the lithium-metal-polyanionic
material is a lithium-ion conductor that operates at or above 4.5 V
versus metallic lithium.

8. The electrode of claim 7, wherein the lithium-metal-polyanionic
material is a lithium-ion conductor that operates at or above 5.0 V
versus metallic lithium.

9. The electrode of claim 1, wherein the lithium-metal-polyanionic
material comprises one or more lithium-metal-phosphate or
lithium-metal-silicate materials.

10. The electrode of claim 9, wherein the lithium-metal-polyanionic
material comprises one or more material selected from the group
consisting of a lithium-nickel-phosphate, a lithium-cobalt-phosphate, a
lithium-magnesium-phosphate, a lithium-nickel-silicate, a
lithium-cobalt-silicate, and a lithium-magnesium-silicate.

11. The electrode of claim 1, wherein the lithium-metal-polyanionic
material is amorphous or poorly crystalline.

12. The electrode of claim 1, wherein the lithium-metal-polyanionic
material comprises a stoichiometric structure, a cation-deficient
structure, an anion-deficient structure, or a combination of two or more
of the foregoing structures.

23. The electrode of claim 22 in which the metal substituent is selected
from a divalent, trivalent or tetravalent ion.

24. The electrode of claim 23, wherein the lithium-metal-polyanionic
material comprises a Li4-xMx/2SiO4 structures in which M
is one or more of Ni, Co, Mg and Zn and 0<x<2.

25. A positive electrode for a non-aqueous lithium cell comprising
lithium-metal-oxide particles containing a lithium metal oxide compound,
the surface of the particles including a lithium-metal-polyanionic
material comprising at least one lithium cation, at least one other metal
cation, and at least one polyvalent metal-free anion and
Li3PO.sub.4.

26. A positive electrode for a non-aqueous lithium cell comprising
lithium-metal-oxide particles containing a lithium metal oxide compound,
the surface of the particles including Li3PO.sub.4.

27. The electrode of claim 1, wherein the lithium-metal-polyanionic
material is fabricated by a sol-gel technique, rf-magnetron sputtering
technique, an atomic layer deposition technique, or a combination of two
or more of the foregoing techniques.

28. An electrochemical cell comprising the electrode of claim 1, a
negative electrode, and an electrolyte therebetween.

29. A battery comprising a plurality of electrochemical cells of claim 28
arranged in parallel, in series, or both.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application claims the benefit of U.S. Provisional Application
Ser. No. 61/148,266, filed on Jan. 29, 2009, which is incorporated herein
by reference in its entirety.

FIELD OF THE INVENTION

[0003]This invention relates to non-aqueous lithium cells and batteries.
Such cells and batteries are used widely to power numerous devices, for
example, portable electronic appliances and medical-, transportation-,
aerospace- and defense systems.

SUMMARY OF THE INVENTION

[0004]This invention relates to positive electrodes (cathodes) for lithium
cells and batteries. More specifically, the invention relates to
lithium-metal-oxide electrodes, notably those having layered-type or
spinel-type structures, or combinations thereof, which are composed of
surface protected lithium-metal-oxide particles. The invention extends to
electrodes in which the oxygen ions of the closed-packed spinel and
layered structures are partially replaced by other anionic species, such
as fluoride ions. A particular embodiment of this invention is that the
surface of the lithium-metal-oxide electrode particles are protected by
one or more lithium-metal-polyanionic materials, for example, a
lithium-metal-phosphate, a lithium-metal-silicate or the like, such as a
lithium-nickel phosphate or a lithium-nickel-silicate, that can act as a
lithium-ion conductor at or above the operating potential of the
lithium-metal-oxide positive electrode, thereby protecting the surface of
the electrode from undesirable effects, such as electrolyte oxidation,
oxygen loss or dissolution. Such surface protection significantly
enhances the surface stability, rate capability and cycling stability of
high capacity lithium-metal-oxide electrodes for lithium-ion cells and
batteries, particularly when charged to high potentials.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005]The invention consists of certain novel features and a combination
of parts hereinafter fully described, illustrated in the accompanying
drawings, it being understood that various changes in the details may be
made without departing from the spirit, or sacrificing any of the
advantages of the present invention.

[0006]FIG. 1 depicts the powder X-ray diffraction patterns of (a)
as-prepared
0.5Li2MnO3.0.5LiNi0.44Co0.25Mn0.31O2 and
(b) a Li--Ni--PO4-treated sample in which * represents peaks from
Li3PO4 or a related phase, and ∇ represents an
unidentified phase.

[0009]FIG. 4 depicts a plot of coulombic efficiency vs. cycle number of
lithium half cells with uncoated and Li--Ni--PO4-coated
0.5Li2MnO3.0.5LiNi0.44Co0.25Mn0.31O2
electrodes at a constant charge and discharge current rate of 0.1
mA/cm2.

[0010]FIG. 5 depicts a plot of capacity vs. cycle number of a Li-ion cell
with a Li--Ni--PO4-coated
0.5Li2MnO3.0.5LiNi0.44Co0.25Mn0.31O2
electrode between 2.0 and 4.5 V (Cycles 1-3 at 0.1 mA/cm2; cycles
3-40 at 0.5 mA/cm2).

[0011]FIG. 6 depicts the electrochemical cycling performance of a lithium
half cell with an uncoated
0.5Li2MnO3.0.5LiNi0.44Co0.25Mn0.31O2
electrode showing (a) the average coulombic efficiency of the cell over
the first 50 cycles, and (b) the capacity retention at various current
rates.

[0012]FIG. 7 depicts the electrochemical cycling performance of a lithium
half cell with a Li3PO4-coated
0.5Li2MnO3.0.5LiNi0.44Co0.25Mn0.31O2
electrode showing (a) the average coulombic efficiency of the cell over
the first 14 cycles, and (b) the capacity retention at various current
rates.

[0013]FIG. 8 depicts the electrochemical cycling performance of a lithium
half cell with a Li2.5Ni0.25PO4-coated
0.5Li2MnO3.0.5LiNi0.44Co0.25Mn0.31O2
electrode showing (a) the average coulombic efficiency of the cell over
the first 13 cycles, and (b) the capacity retention at various current
rates.

[0014]FIG. 9 depicts the electrochemical cycling performance of a lithium
half cell with a Li1.5Ni0.75PO4-coated
0.5Li2MnO3.0.5LiNi0.44Co0.25Mn0.31O2
electrode showing (a) the average coulombic efficiency of the cell over
the first 9 cycles, and (b) the capacity retention at various current
rates.

[0015]FIG. 10 depicts a schematic representation of an electrochemical
cell.

[0016]FIG. 11 depicts a schematic representation of a battery consisting
of a plurality of cells connected electrically in series and in parallel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017]As used herein, the term "lithium-metal-oxide" and grammatical
variations thereof, refers to lithium metal oxide compounds, which can
optionally include lithium metal oxides in which some oxygen ions have
been replaced by other anionic species, such as fluoride ions. The term
"lithium-metal-polyanionic material" and grammatical variations thereof,
refers to materials having a general formula that comprises at least one
lithium cation, at least one other metal cation (e.g., a Ni or Co
cation), and at least one polyanion (e.g., phosphate, silicate,
tungstate, molybdate, etc.).

[0018]Conventional lithium-ion battery cathodes, such as layered
LiCoO2, spinel LiMn2O4, olivine LiFePO4 and
compositional variations thereof, do not deliver sufficient
electrochemical capacity and power to satisfy the driving range
requirements for plug-in hybrid-electric vehicles (PHEVs) and
all-electric vehicles. Moreover, there is a growing demand to increase
the energy and power of lithium-ion batteries for other wide-ranging
applications, such as portable electronic devices, medical devices,
aerospace and defense applications and for stand-alone energy storage.
Conventional electrode materials such as LiCoO2, LiMn2O4
and LiFePO4 typically deliver capacities of 100-160 mAh/g between
4.2 and 3.0 V at moderate to high rates. Layered LiMO2 compounds, in
which M is selected typically from electroactive metal cations, such as
Mn, Co, Ni, and additional stabilizing cations such as Li and Al, provide
the best opportunity to increase the electrode capacity and hence the
energy of lithium-ion cells and batteries, because they offer a maximum
capacity of up to approximately 280 mAh/g at potentials greater than 3.0
V vs. metallic lithium. However, the highly oxidizing character and
instability of lithium-metal-oxide electrodes, in particular, at low
lithium loadings, as well as solubility effects, has limited the extent
to which this high capacity can be realized, particularly at high rates.

[0019]Several efforts have already been made in the past to overcome the
stability and solubility problems associated with lithium-metal-oxide
electrodes. For example, considerable success has been achieved by
stabilizing electrodes by pre-treating the electrode powders with oxide
additives such as Al2O3 or ZrO2 obtained from metal
alkoxide precursors such as solutions containing aluminum ethylhexanoate
diisopropoxide (Al(OOC8H15)(OC3H7)2 or zirconium
ethylhexanoate isopropoxide
(Zr[(OOC8H15)2(OCH3H7)2]) as described, for
example, by J. Cho et al. in Chemistry of Materials, Volume 12, page 3788
(2000) and J. Cho et al. in Electrochemical and Solid State Letters,
Volume 4 No. 10, page A159 (2001), respectively, or a zirconium oxide,
polymeric precursor or zirconium oxynitrate
(ZrO(NO3)2.xH2O) as described by Z. Chen et al. in
Electrochemical and Solid State Letters, Volume 5, No. 10, page A213
(2002), prior to the fabrication of the final electrode thereby making
the surface of the LiCoO2 particles more resistant to electrolyte
attack, cobalt dissolution or oxygen loss effects. Colloidal particles
have also been used to protect lithium-metal-oxide electrode surfaces, as
described by Oesten et al. in US Patent Application 2001/0046628, and by
Kim et al. in the Journal of the Electrochemical Society, Volume 151,
page A1755 (2004). More recently, Manthiram et al. in the Journal of The
Electrochemical Society, Volume 155, page A635 (2008) and Sun et al. in
the Journal of The Electrochemical Society, Volume 154, page A168 (2007),
and have shown that AlPO4 and AlF3 coatings, respectively,
improve the electrochemical cycling stability of lithium-metal oxide
electrodes. Despite the success of improving the performance of
lithium-ion cells by coating individual electrode particles with
colloidal Al2O3 or ZrO2 particles or with AlPO4, the
coatings themselves either generally impede lithium diffusion in and out
of the layered electrode structure during electrochemical discharge and
charge, lowering the rate capability of the cells. On the other hand,
although AlF3 coatings reported by Sun et al., as referenced above,
were shown by impedance spectroscopy to increase the rate capability of
Li[Ni1/3Co1/3Mn1/3]O2 electrodes, when charged to 4.6
V, the capacity delivered by the electrodes was only about 160 mAh/g when
discharged at a C/1 rate.

[0020]The loss of oxygen from lithium metal oxide electrodes, such as
layered LiCoO2 and LiNi1-yCoyO2 electrodes can
contribute to exothermic reactions with the electrolyte and with the
lithiated carbon negative electrode, and subsequently to thermal runaway
if the temperature of the cell reaches a critical value. Further
improvements in the composition and structure of the surfaces of
lithium-metal oxide electrodes are therefore still required to protect
the intrinsic capacity of the electrode from decay and to improve the
overall performance and safety of lithium-ion cells without compromising
the rate capability of the electrode.

[0021]Lithium metal oxides with a spinel-type structure are particularly
attractive lithium-ion battery electrodes for high-power applications. Of
particular significance is the lithium-manganese-oxide spinel,
LiMn2O4, and its cation-substituted derivatives,
LiMn2-xMxO4, in which M is one or more metal ions
typically a monovalent or a multivalent cation such as Li+,
Mg2+ and Al3+, as reported by Gummow et al. in U.S. Pat. No.
5,316,877 and in Solid State Ionics, Volume 69, page 59 (1994). It is
well known that LiMn2O4 and metal-substituted
LiMn2-xMxO4 spinel electrodes are chemically unstable in a
lithium-ion cell environment, particularly at high potentials and/or when
the cell operating temperature is raised above room temperature, when
manganese ions from the spinel electrodes tend to dissolve in the
electrolyte. This process is believed to contribute to the capacity loss
of the cells at elevated temperatures. Moreover, the removal of all the
lithium from LiMn2-xMxO4 spinel electrodes, notably
LiMn2O4 (x=0), yields a Mn2-xMxO4 (MnO2,
x=0) component, which itself is a strong oxidizing agent. The surface of
such delithiated spinel electrodes can have a high oxygen activity,
thereby possibly inducing unwanted oxidation reactions with the
electrolyte. Although considerable progress has been made to suppress the
solubility and high-temperature performance of spinel electrodes and to
improve their stability by cation doping, as described for example by
Gummow et al. in U.S. Pat. No. 5,316,877, or by forming oxyfluoride
compounds as described by Amatucci et al. in the Journal of the
Electrochemical Society, Volume 149, page K31 (2002) and by Choi et al.
in Electrochemical and Solid-State Letters, Volume 9, page A245-A248
(2006), or by surface coatings as described by Kim et al. in the Journal
of the Electrochemical Society, Volume 151, page A1755 (2004), these
treatments have not yet entirely overcome the cycling instability of
cells containing manganese-based spinel electrodes.

[0022]Considerable progress has been made over recent years to stabilize
cubic-close-packed layered lithium-metal-oxide electrode systems by using
lithium- and manganese-rich composite electrode structures,
xLi2MnO3.(1-x)LiMO2 in which M is, for example, Mn, Ni,
and/or Co, as described in U.S. Pat. No. 6,677,082 and U.S. Pat. No.
6,680,143, and by Kim et al. in Chemistry of Materials, Volume 16, page
1996 (2004), and by Thackeray et al. in the Journal of Materials
Chemistry, Volume 17, page 3112 (2007). These electrodes can deliver
essentially all their theoretical capacity (240-250 mAh/g) at relatively
low rate, for example C/24, as reported by Johnson et al. in
Electrochemistry Communications, Volume 6, page 1085 (2004). Composite
electrode structures containing cubic-close-packed layered- and spinel
components, such as xLi2MnO3.(1-x)LiMn2-xMxO4 in
which M is a metal cation selected preferably from Li, Ni, Co, Al and Mg
have also been disclosed, as described for example, by Johnson et al. in
Electrochemistry Communications, Volume 7, page 528 (2005), and by
Thackeray et al. in the Journal of Materials Chemistry, Volume 15, page
2257 (2005). These composite electrodes form because of the structural
compatibility of the cubic-close-packed oxygen arrays of the individual
lithium-metal-oxide components. The integrated structures are highly
complex and are often characterized by complicated cation arrangements
with short range order.

[0023]When the manganese- and nickel ions are nearest neighbors in layered
and spinel electrode structures and in the composite electrode structures
described above, they tend to adopt tetravalent and divalent oxidation
states, respectively. The lithium and transition metal ions are
distributed in highly complex arrangements; the Li+ and Mn4+
ions are arranged in small localized regions to give the structure
Li2MnO3-like character. Composite layered materials can be
represented either in two-component notation,
xLi2MnO3.(1-x)LiMO2, in which the close-packed
Li2MnO3 and LiMO2 components are structurally integrated
or, alternatively, when normalized in standard layered (rock salt)
notation, as Li.sub.(2+2x)/(2+x)Mn2x/(2+x)M.sub.(2-2x)/(2+x)O2.
The Li2MnO3 component that supplies surplus lithium to the
layered structure plays a critical role in stabilizing the electrode
structure at low lithium loadings; on lithium extraction, lithium ions in
the transition metal layers diffuse into the lithium depleted layers to
provide sufficient binding energy to maintain the integrity of the
close-packed oxygen array.

[0024]Electrochemical extraction of lithium from
xLi2MnO3.(1-x)LiMO2 during the initial charge occurs in
two steps. When taken to completion above 4.6 V vs. Li0, the ideal
reactions can be represented:

LiMO2→MO2+Li++e.sup.- (1)

Li2MnO3→MnO2+2Li++1/2O2+2e.sup.- (2)

[0025]Despite the removal of lithium and oxygen from the Li2MnO3
component, the layered character of the residual MnO2 component
remains remarkably intact. The highly oxidizing nature of both the
MO2 and MnO2 components, however, can result in oxygen loss at
the particle surface, particularly when M=Co and/or Ni, thereby damaging
the electrode surface; electrolyte oxidation can also occur at these high
potentials. These factors limit the rate at which lithium can be
reinserted into the charged, high-capacity xMnO2.(1-x)MO2
electrode. These electrodes also tend to lose capacity on cycling.

[0026]Attempts to stabilize cubic-close-packed layered as well as spinel
lithium-metal-oxide structures by integrating them with a
hexagonal-close-packed olivine structure have thus far been unsuccessful.
For example, R. M. Ward et al., in the U.S. Department of Energy Journal
of Undergraduate Research, Volume VI, page 91 (2006), demonstrated in
their studies of reactions of LiNiPO4 (olivine) and
LiNi0.5Mn0.5O2 (layered) and
Li[Ni0.5Mn1.5]O4 (spinel) precursors that the X-ray
diffraction data of the products showed a complex mixture of phases,
rather than structurally-integrated
LiNiPO4--LiNi0.5Mn0.5O2 (`olivine-layered`) and
LiNiPO4--Li[Ni0.5Mn1.5]O4 (olivine-spinel) composite
materials. Thackeray et al. reported at the 25th International
Battery Seminar and Exhibit, Fort Lauderdale, Fla., Mar. 17-20, 2008 and
at the International Workshop on Fundamentals of Lithium-based Batteries,
Schloss Ringberg, Tegernsee, Germany, Nov. 23-28, (2008) that high
resolution transmission electron diffraction of olivine LiNiPO4 and
spinel LiNiVO4 precursors after reaction at elevated temperatures
existed as discrete olivine and spinel phases, respectively, rather than
as an integrated structure.

[0027]Despite the apparent inability of olivine compounds (LiMPO4,
M=Mn, Fe, Co, Ni) with a PO43- polyanionic framework to form
integrated, epitaxially-grown structures with layered and spinel lithium
metal oxides, it has now been discovered that depositing a
lithium-metal-phosphate material, for example, a Li--Ni--PO4
material onto the surface of a layered lithium metal oxide using a
sol-gel method followed by a heat-treatment step, significantly enhances
the surface stability, rate capability and cycling stability of
lithium-metal-oxide electrodes, particularly when charged to high
potentials, thereby allowing high capacities to be delivered at improved
rates. The principles of this invention, therefore, can be extended to
other types of lithium-metal-oxide structures that suffer from chemical
and electrochemical instabilities at high potentials, for example, the
family of lithium-manganese-oxides with a spinel-type structure, and
substituted variations thereof. These advances are particularly relevant
to lithium-ion batteries that power applications such as cell phones,
laptop computers, power tools, hybrid-, plug-in hybrid- and all-electric
vehicles for which the demand for higher energy and power batteries is
growing. These new advances were first disclosed, in concept and in
practical data, by Thackeray et al. at the 25th International
Battery Seminar and Exhibit, Fort Lauderdale, Fla., Mar. 17-20, 2008 and
at the International Workshop on Fundamentals of Lithium-based Batteries,
Schloss Ringberg, Tegernsee, Germany, Nov. 23-28 (2008), respectively.

[0028]This invention relates to positive electrodes (cathodes) for lithium
cells and batteries. More specifically, the invention relates to surface
protected lithium-metal-oxide electrodes, notably those having
layered-type or spinel-type structures, or combinations thereof. The
invention extends to electrodes in which the oxygen ions of the
closed-packed spinel and layered structures are partially replaced by and
contain other anionic species, such as fluoride ions. In one embodiment,
the lithium-metal-oxide electrode is comprised of a one or more layered
compounds, represented by the general formula,
xLi2M'O3.(1-x)LiMO2 in which M' comprises one or more
metal ions with an average tetravalent oxidation state, and M comprises
one or more metal ions with an average trivalent oxidation state, as
described and defined more fully by Thackeray et al. in U.S. Pat. Nos.
6,677,082 and 6,680,143. In a preferred embodiment, the M' comprises Mn,
and M comprises one or more metal ions, selected preferably from Mn, Ni
and Co. A second embodiment of this invention is that the surface of the
lithium-metal-oxide electrode particles is protected by, and comprised
of, one or more lithium-metal-polyanionic materials, such as a
lithium-metal-phosphate, a lithium-metal-silicate or the like, in which
the polyanion is comprised of a negatively charged species that contains
more than one atom type, for example WO3.sup.-, MoO3.sup.-,
SO42-, PO43-, SiO44-. In a preferred
embodiment, the negatively charged species are metal-free polyanions,
such as PO43- and SiO44-. In another preferred
embodiment, the lithium-metal-polyanionic materials can act as
lithium-ion conductors at or above the operating potential of the
lithium-metal-oxide positive electrode to provide access of the lithium
ions from the electrolyte to the electrode during discharge, and
vice-versa during charge, while simultaneously protecting the surface of
the electrode from undesirable effects, such as electrolyte oxidation,
oxygen loss or dissolution. Such surface protection significantly
enhances the surface stability, rate capability and cycling stability of
the lithium-metal-oxide electrodes, particularly when charged to high
potentials.

[0029]In another embodiment of this invention, the positive electrodes are
protected by a modified surface, surface layer or coating comprising a
lithium-metal-polyanionic material that is stable at and/or above the
operating electrochemical potential of the lithium-metal-oxide electrode.
It is desirable that the modified surface, surface layer or coating
should act predominantly or exclusively as a stable lithium-ion conductor
that operates preferably at or above 4 V, more preferably at or above 4.5
V and most preferably at or above 5.0 V versus metallic lithium, thereby
allowing the electrode to operate repeatedly at high rates without
subjecting the modified surface, surface layer or coating to potentially
damaging redox reactions that might affect the electrochemical properties
of the electrode. The lithium-metal-polyanionic material may be comprised
of lithium, one or more metals, and one or more polyanions. In addition,
the lithium-metal-polyanionic material may be amorphous or,
alternatively, it may be poorly crystalline or strongly crystalline with
either stoichiometric structures or cation and/or anion defect
structures. Therefore in this embodiment, the positive electrode
comprises lithium-metal-oxide particles, the surface of the particles
comprising a lithium-metal-polyanionic material, the general formula of
which comprises at least one lithium cation, at least one other metal
cation, and at least one polyanion.

[0030]The lithium-metal-polyanionic material is comprised preferably of
one or more lithium-metal-phosphate or lithium-metal-silicate materials,
for example, those selected from the family of lithium-nickel-phosphate,
lithium-cobalt-phosphate, lithium-magnesium-phosphate,
lithium-nickel-silicate, lithium-cobalt-silicate, and
lithium-magnesium-silicate materials. In a further preferred embodiment,
the lithium-metal-polyanionic material is comprised of a
lithium-nickel-phosphate, lithium-cobalt-phosphate,
lithium-magnesium-phosphate, lithium-nickel-silicate,
lithium-cobalt-silicate, and lithium-magnesium-silicate compositions and
structures, including stoichiometric- or defect olivine-related
LiMPO4 structures (for example, M=Ni, Co, Mg, Zn),
Li3PO4-related structures as well as metal-substituted
Li3PO4-related structures, such as defect
Li3-xMx/2PO4 (for example, M=Ni, Co, Mg, Zn; 0<x<2)
structures, defect

Li3-xMx/3PO4 (for example, M=Al, Ga, and La; 0<x<3)
and Li2MSiO4-related structures such as stoichiometric
Li2NiSiO4 and Li2CoSiO4 and defect
Li2-xMSiO4 structures. In the stoichiometric and defect
compounds of this invention, such as LiMPO4,
Li3-xMx/2PO4, Li3-xMx/3PO4,
Li2MSiO4 and Li2-xMSiO4 compositions and structures,
the M cations may be partially or completely substituted by other metal
cations, for example, divalent cations, such as Mg2+ or Zn2+
ions, and trivalent cations, such as Al3+ ions, and tetravalent
cations, such as Zr4+ ions, that can also form lithium-ion
conducting, solid electrolyte compounds. Of particular significance is
the advantage that lithium-metal-polyanionic materials containing
divalent metal cations, such as LiNiPO4 and LiCoPO4, can remain
stable and electrochemically inactive to lithium extraction to a high
electrochemical potential of approximately 5 V vs. lithium metal. The
applicants believe that a particular advantage of having stable divalent
nickel ions in the modified surface, surface layer or coating may aid to
stabilize manganese-based lithium-metal-oxide electrodes because any
Ni2+--Mn4+ nearest neighbor interactions would contribute
further to stabilizing the lithium-metal-oxide electrode surface by
suppressing surface Mn3+ species and manganese solubility.

[0031]The lithium-metal-polyanionic material of this invention may also
include Li3PO4 as a component of the protective layer. In this
respect, Li3PO4 may either be the major component (>50%) or
the minor component (<50%) of the surface structure or, alternatively,
it may be used entirely as the protective surface layer or coating of the
lithium-metal-oxide electrode.

[0032]In a further embodiment, the invention is extended to include
Li4SiO4-related compositions and structures and substituted
compositions and structures, for example, metal-substituted, defect
Li4-xMx/2SiO4 structures in which M is one or more
divalent cations such as Ni2+, Co2+, Mg2+ and Zn2+
and 0<x<2. In metal-substituted Li4SiO4 structures, the
substituted M cations may alternatively be comprised of trivalent
cations, such as Al3+ ions, or tetravalent cations, such as
Zr4+ ions, that can form lithium-ion conducting compounds.

[0033]The invention includes experimental procedures and treatments for
depositing the protective layers onto, or coating the
lithium-metal-oxide-electrodes. Examples include, in particular, standard
sol-gel, rf-magnetron sputtering and/or atomic layer deposition
techniques. Because the precise nature of the modified surface, surface
layer or coating is not known, the terms `surface treatment` and `surface
coating` as disclosed in this specification are used synonymously and
interchangeably.

[0034]The following examples describe the principles of the invention as
contemplated by the inventors, but they are not to be construed as
limiting examples.

Example 1

[0035]A mildly fluorinated
0.5Li2MnO3.0.5LiNi0.44Co0.25Mn0.31O2
(Li1.200Mn0.524Ni0.176Co0.100O2) electrode was
prepared from a powdered precursor as described previously by Kang et al.
in the Journal of the Electrochemical Society, Volume 153, page A1186
(2006). A lithium nickel phosphate protective material was applied to the
electrode powder using a sol-gel method, by immersing and treating the
powder in an acidic solution (pH<4) of lithium acetate, nickel nitrate
and ammonium dihydrogen phosphate with glycolic acid as a chelating agent
using a Li:Ni:P ratio of about 1:1:1. Nitric acid was used to control the
pH of the solution to prevent precipitation. The electrode powder was
stirred continuously in the solution and heated slowly to dryness. The
resulting Li--Ni--PO4-coated powder product was finally heated at
about 550° C. for about 6 hours in air.

[0036]Powder X-ray diffraction (XRD) patterns of Li--Ni--PO4-coated
samples were collected on a Siemens D5000 diffractometer (CuKα)
between 10 and 80° 2θ. Coin-type cells (2032, Hohsen) were
constructed from the coated powder in an argon-filled glovebox (<5 ppm
O2 and H2O). The cathode consisted of 80 wt % of the coated
oxide powder, 10 wt % carbon, and 10 wt % polyvinylidene difluoride
(PVDF) binder on aluminum foil. The anode was either metallic lithium or
graphite (MAG-10, Hitachi with 8 wt % PVDF) on copper foil. The
electrolyte was 1.2M LiPF6 in a 3:7 mixture of ethylene carbonate
and ethylmethyl carbonate. For the cycling experiments, cells were
galvanostatically charged and discharged between 2.0 and 4.6 V (2.0 and
4.5 V for the Li-ion cells) at different currents (0.1-2.0 mA/cm2)
and trickle charged at 4.6 V for 3 hours. For the rate tests, lithium
cells were charged to 4.6 V at 0.1 mA/cm2 with a trickle charge at
4.6 V for 3 hours; cells were discharged to 2.0 V at 0.1 to 1.0
mA/cm2 with three cycles at each rate. Electrochemical experiments
were conducted at room temperature and duplicated to check
reproducibility.

[0037]The powder X-ray diffraction patterns of the parent, mildly
fluorinated
0.5Li2MnO3.0.5LiNi0.44Co0.25Mn0.31O2
material and the Li--Ni--PO4-coated sample are shown in FIG. 1
(patterns (a) and (b), respectively). The pattern of the uncoated sample
(FIG. 1, pattern (a)) is typical of layered
xLi2MnO3.(1-x)LiMO2 materials; it shows the characteristic
weak ordering peaks from the Li2MnO3-type component at
21-25° 2θ. The XRD pattern of the Li--Ni--PO4-coated
sample (FIG. 1, pattern (b)) is essentially identical to the parent
compound; it shows a minor amount of Li3PO4 and/or structurally
related Li3-xNix/2PO4, and a few additional weak,
unidentified peaks. The detailed nature of the surface structure is
currently unknown.

[0038]It is already known that fluorination improves the room temperature
cycling stability of layered lithium-metal-oxide electrodes and that the
fluorine component resides predominantly at the particle surface, as
described for example by Kang et al. in the Journal of Power Sources,
Volume 146, page 654 (2005), Park et al. in the Journal of Power Sources
Volume 178, page 826 (2008), and Kim et al. in the Journal of the
Electrochemical Society, Volume 152, page A1707 (2005). Surface
fluorination of
0.1Li2MnO3.0.9LiMn0.256Ni0.372Co0.372O2
(Li1.048[Mn0.333Ni0.333Co0.333]0.952O2)
electrodes using mildly acidic fluorinated solutions improves their rate
capability as described by Kang et al. in the Journal of the
Electrochemical Society, Volume 155, page A269 (2008); in this instance,
at 1.0 mA/cm2, which approximates a C/1 rate, the parent electrode
yielded approximately 160 mAh/g, whereas the fluorinated electrode
yielded 175 mAh/g.

[0039]The voltage profiles of the initial charge/discharge cycle of
lithium half cells with the uncoated and Li--Ni--PO4-coated
0.5Li2MnO3.0.5LiNi0.44Co0.25Mn0.31O2
electrodes of this invention, obtained at 0.1 mA/cm2 (<C/10 rate)
are shown in FIG. 2. The initial charge profiles and capacity
(approximately 295 mAh/g) of the cells are similar, indicating lithium
extraction first from the LiNi0.44Co0.25Mn0.31O2
component between 3.0 and approximately 4.4 V, followed by lithium
extraction and oxygen loss (net loss Li2O) between 4.4 and 4.6 V. On
discharge, the Li--Ni--PO4-coated electrode delivered approximately
260 mAh/g vs. the approximately 240 mAh/g from the uncoated electrode,
the coated electrode exhibiting a higher, first-cycle coulombic
efficiency of 87% compared to the uncoated electrode (81%). The
Li--Ni--PO4-coated electrodes delivered their capacity at potentials
above that of the parent uncoated electrode (FIG. 2), implying that the
kinetics of the electrochemical reaction was faster in the
Li--Ni--PO4-coated electrodes of the invention.

[0040]The relative rate capability of uncoated and Li--Ni--PO4-coated
electrodes is shown in FIG. 3, graph (a). The data indicate that the
Li--Ni--PO4-coated electrode is significantly more tolerant to
higher discharge than the uncoated parent
0.5Li2MnO3.0.5LiNi0.44Co0.25Mn0.31O2
electrode and shows less polarization. The beneficial effect of the
surface Li--Ni--PO4 material is more pronounced at high discharge
rates, the coated electrode showing a higher capacity retention at 1.0
mA/cm2 (80%) relative to the capacity at 0.1 mA/cm2 than the
uncoated electrode (70%). The corresponding capacity vs. cycle number
plots of these lithium cells, for charge/discharge currents of 0.1, 1.0
and 2.0 mA/cm2, emphasize the excellent and surprisingly superior
cycling stability of the coated electrodes (FIG. 3, graph (b)).

[0041]FIG. 4 shows a plot of coulombic efficiency vs. cycle number for
lithium half cells with uncoated and Li--Ni--PO4-treated
0.5Li2MnO3.0.5LiNi0.44Co0.25Mn0.31O2
electrodes (referenced ANLCC) at a constant charge and discharge current
rate of 0.1 mA/cm2. The data highlight the superior coulombic
efficiency on repeated cycling of the treated electrodes, in accordance
with the principles of this invention.

[0042]FIG. 5 shows the capacity vs. cycle number plot of a full Li-ion
cell in which the cycling stability of the Li--Ni--PO4-coated
electrode was evaluated against a graphite anode. The cell was cycled at
0.1 mA/cm2 (C/11) for the first three cycles between 4.5 and 2.0 V,
and at 0.5 mA/cm2 (C/2) for the following 37 cycles. The results are
consistent with the half cell data in FIG. 3, graphs (a) and (b). At C/2,
the treated electrode provides approximately 225 mAh/g at room
temperature which, from a rate standpoint, is considerably superior to
data reported by Kang et al. in the Journal of the Electrochemical
Society, Volume 155, page A269 (2008) for fluorinated, but uncoated,
xLi2MnO3.(1-x)LiMO2 electrodes.

Example 2

[0043]Coatings were applied to mildly fluorinated electrode materials
0.5Li2MnO3.0.5LiNi0.44Co0.25Mn0.31O2
(Li1.200Mn0.524Ni0.176Co0.100O2) following the
same treatment to the powders as described in Example 1. In these
experiments, solutions containing various amounts of lithium, nickel and
phosphate ions were used in accordance with the formula
Li3-2xNixPO4 for x=0, 0.25 and 0.75 such that a 2 mole
percent coating was applied. In addition, an electrode sample with a
coating of 2 mole percent Li3PO4 was also prepared, for
comparison. In the final step, the resulting Li--Ni--PO4-coated
electrode products were dried by heating at about 550° C. for
about 6 hours in air. The electrochemical properties of these electrode
products were evaluated to determine, in particular, the effect that the
various surface treatments or surface coatings had on coulombic
efficiency and capacity, when cells were cycled at various current rates.

[0044]Electrochemical cells were assembled as described in Example 1.
Representative data for the various cells with an uncoated electrode
(x=0), a Li3PO4-coated electrode, and electrodes coated with
Li2.5Ni0.25PO4 (x=0.25) and Li1.5Ni0.75PO4
(x=0.75) compositions are shown in FIGS. 6, 7, 8 and 9, respectively. The
conditions under which the cells were cycled, for example, the operating
voltage window, current rates, and average capacities at a particular
current rate, are provided in Table 1. For these comparative experiments,
the current rate was measured in terms of mA/g, rather than mA/cm2,
for greater accuracy.

[0045]FIGS. 6-9 clearly emphasize the significant improvement of the
coulombic efficiency of cells with coated electrodes over those with an
uncoated electrode. The uncoated electrode, while providing good
electrochemical cycling stability over 50 cycles, operated, on average,
with 98.7 coulombic efficiency (FIG. 6, graph (a)); it provided an
average capacity of about 175 mAh/g at 150 mA/g (FIG. 6, graph (b)). The
Li3PO4-coated electrode, provided an excellent coulombic
efficiency (100.2%), but showed slightly lower capacity at a 150 mA/g
rate than the uncoated electrode (FIG. 7, graphs (a) and (b)). The two
Li3-2xNixPO4-coated electrodes also provided outstanding
coulombic efficiencies of 100.1 and 100.0% for x=0.25 and x=0.75 i.e.,
significantly higher than the uncoated electrode, respectively, thereby
demonstrating the advantages of the coated electrodes of this invention.
They delivered capacities of 184 and 193 mAh/g, respectively, at 150
mA/g, which is approximately a C/1 rate (FIGS. 8 and 9, graphs (a) and
(b)). These results indicate that Li3PO4 provides the most
resistive layer or coating, and that the rate capability of the
Li3-2xNixPO4 layers or coatings increases as a function of
increasing Ni content, x.

[0046]This invention, therefore, relates to surface-protected
lithium-metal-oxides that can be used as positive electrodes for a
non-aqueous electrochemical lithium cell as shown schematically in FIG.
10, the cell represented by the numeral 10 having a negative electrode 12
separated from a positive electrode 16 by an electrolyte 14, all
contained in an insulating housing 18 with suitable terminals (not shown)
being provided in electronic contact with the negative electrode 12 and
the positive electrode 16. Binders and other materials normally
associated with both the electrolyte and the negative and positive
electrodes are well known in the art and are not described herein, but
are included as is understood by those of ordinary skill in this art.
FIG. 11 shows a schematic illustration of one example of a battery in
which two strings of electrochemical lithium cells, described above, are
arranged in parallel, each string comprising three cells arranged in
series.

[0047]The use of the terms "a" and "an" and "the" and similar referents in
the context of describing the invention (especially in the context of the
following claims) are to be construed to cover both the singular and the
plural, unless otherwise indicated herein or clearly contradicted by
context. The terms "comprising," "having," "including," and "containing"
are to be construed as open-ended terms (i.e., meaning "including, but
not limited to,") unless otherwise noted. Recitation of ranges of values
herein are merely intended to serve as a shorthand method of referring
individually to each separate value falling within the range, unless
otherwise indicated herein, and each separate value is incorporated into
the specification as if it were individually recited herein. All methods
described herein can be performed in any suitable order unless otherwise
indicated herein or otherwise clearly contradicted by context. The use of
any and all examples, or exemplary language (e.g., "such as") provided
herein, is intended merely to better illuminate the invention and does
not pose a limitation on the scope of the invention unless otherwise
claimed. No language in the specification should be construed as
indicating any non-claimed element as essential to the practice of the
invention.

[0048]Preferred embodiments of this invention are described herein,
including the best mode known to the inventors for carrying out the
invention. Variations of those preferred embodiments may become apparent
to those of ordinary skill in the art upon reading the foregoing
description. The inventors expect skilled artisans to employ such
variations as appropriate, and the inventors intend for the invention to
be practiced otherwise than as specifically described herein.
Accordingly, this invention includes all modifications and equivalents of
the subject matter recited in the claims appended hereto as permitted by
applicable law. Moreover, any combination of the above-described elements
in all possible variations thereof is encompassed by the invention unless
otherwise indicated herein or otherwise clearly contradicted by context.

Patent applications by Michael M. Thackeray, Naperville, IL US

Patent applications by Sun-Ho Kang, Naperville, IL US

Patent applications by UCHICAGO ARGONNE, LLC

Patent applications in class Nickel component is active material

Patent applications in all subclasses Nickel component is active material